Smart windows are windows that can be user-controlled to block off some or all light from entering the home. As of 2014, a smart window can be built using either suspended particle devices or liquid crystals to achieve the light-blocking effect.
In both SPD and liquid-crystal smart windows, the material is inserted between two panes of glass. The inserted material allows the user to control the amount of light that enters the house through the window. Smart windows using SPD need a constant flow of electricity to maintain transparency. When electricity is halted or reduced, an SPD window will begin to darken and become opaque. Liquid-crystal smart windows also require an electrical current.